1. Field of the Invention
The present invention relates to an X-ray photography apparatus.
2. Description of the Background Art
Conventionally what is called intraoral radiography (dental radiography), in which an X-ray detection means (such as an X-ray film and an X-ray sensor panel) is set in the oral cavity of a patient to photograph part of a row of teeth or gums, is performed in X-ray photography of the dental field. Because only a local region is irradiated with an X-ray in the intraoral radiography, advantageously the photographing is simply performed and X-ray exposure is reduced. However, it is necessary to dispose in advance the X-ray detection means in the oral cavity of the patient, and thus a large burden is placed on the patient.
Therefore, Japanese Patent Application Laid-Open No. 2007-136163 has made a proposal that panoramic photography is performed using a panoramic image photographing device and a tomographic image of part of the row of teeth or the gums is acquired using the frame data obtained by the panoramic photography. In the panoramic photography, the X-ray is detected using an X-ray detector disposed outside the head of a patient. For this reason, the intraoral radiography can be performed in a pseudo manner while the burden on the patient is reduced.
The conventional intraoral radiography is roughly divided into two: an intraoral radiography by a paralleling technique and an intraoral radiography by a bisecting angle technique. In the intraoral radiography performed by the paralleling technique, part of the row of teeth or the gums is irradiated with the X-ray such that the X-ray is orthogonal to the tooth axis of the tooth. In the case that part of the row of teeth or the gums is irradiated with the X-ray such that the X-ray is not orthogonal to the tooth axis, there is a risk that an X-ray image becomes such an image that the tooth is obliquely looked down from above or that the tooth is obliquely looked up from below, and possibly the image of the tooth is taken shorter (or longer) than in reality. The X-ray image is not desirable in an image diagnosis. Advantageously the problem is eliminated in the intraoral radiography by the paralleling technique.
On the other hand, in the intraoral radiography performed by the bisecting angle technique, part of the row of teeth or the gums is irradiated with the X-ray such that the X-ray is orthogonal to the line bisecting the angle formed by the tooth axis of the tooth and an X-ray film. In this case, because the X-ray is obliquely incident to the target tooth, the image of the tooth is taken shorter than in reality. However, the length of the tooth viewed in the X-ray irradiation direction is equal to the length of the image of the tooth taken in the X-ray film. Therefore, in the intraoral radiography by the bisecting angle technique, advantageously an observer can directly observe the tooth viewed in the X-ray irradiation direction as an X-ray photograph.
In the X-ray photography apparatus disclosed in Japanese Patent Application Laid-Open No. 2007-136163, the X-ray irradiation angle is not particularly changed during the panoramic photography. Therefore, the X-ray irradiation angle is undesirable with respect to the interest tooth, and the tomographic image equivalent to the X-ray image obtained by the intraoral radiography is hardly acquired.